I. Technical Field
The present invention relates to electronic thermostats. More specifically, the present invention relates to a novel electronic thermostat with a switched power converter for providing continuous power to the thermostat from a gas heater's activation circuit.
II. Background Art
The common gas household heating furnace is typically controlled by a room thermostat located remotely from the furnace. The thermostat, generally located in the room where the temperature is to be controlled, in conjunction with the heater activation circuit, integrated within the furnace, control the burning of gas within the furnace. The heater activation circuit, in response to the state of the thermostat, controls the gas flow into the furnace through a solenoid controlled gas valve.
Power for the heater activation circuit is typically provided through a step-down transformer that receives the nominal 120 volt, 60 Hz AC power at the step-down transformer primary winding. The step-down transformer provides at a secondary winding an output to the heater activation circuit of 24 volt, 60 Hz AC power. Typically the thermostat and the coil of the gas valve solenoid are series connected with the secondary of the step-down transformer.
The thermostat is typically a bistate control element which permits manual or automatic control of room temperatures. When the thermostat is in one state, i.e. room temperature has met or exceeded the predetermined temperature setting of the thermostat, the gas valve solenoid is deenergized under the control of the thermostat. As a result, the flow of gas which burns within the furnace is cut off and heating is inhibited.
When the room temperature drops below the predetermined temperature setting, the thermostat changes state. This change in states of the thermostat results in the energizing of the gas valve solenoid. In this energized condition the thermostat allows sufficient current to flow through the coil of the gas valve solenoid to fully energize the solenoid. Energizing the gas valve solenoid permits gas flow into the furnace for burning therein. Thermostats are typically constructed such that in this condition a short circuit is provided across the thermostat. As a result, full voltage of the step-down transformer secondary appears across the coil of the gas valve solenoid, resulting in energization of the solenoid. However, with a short circuit appearing at the thermostat, no voltage is available to the thermostat itself.
In applications where electronic circuitry is used within the thermostat for temperature sensing and heater activation circuit controlling, uninterrupted power must be provided to the electronic circuitry for continuous operation. One attempt at providing continuous power to the electronic thermostat has been the use of batteries within the thermostat. Such schemes have included either replaceable batteries or the incorporation of rechargeable batteries, individually, or in combination with a battery charger circuit integrated within the thermostat. In both cases, the batteries must be replaced at periodic intervals and the cost of the thermostat increases due to the increased circuit complexity and battery cost.
Other approaches at providing power to the thermostat from the heater activation circuit have been attempted. One such approach utilized voltage/current switching through silicon controlled rectifiers in series with heater activation circuit. This scheme resulted in highly distorted voltage and current waveforms in the heater activation circuit. These distorted waveforms may release electromagnetic radiation into the household environment, and throughout the electrical power network to other households. These distortions may endanger the lives of individuals using heart pacing devices in addition to producing radio and television interference. In addition to the distorted voltage and current waveforms the attempted approach does not provide for the control of the activation of the gas valve solenoid. This control is still utilizing the mechanical bimetal element of the mechanical thermostat.
It is therefore an object of the present invention to provide a novel and improved electronic thermostat incorporating a switched power converter which permits continuous power to be provided to the thermostat from a heater activation circuit.
It is yet another object of the present invention to provide an electronic thermostat with active switching of the heater activation circuit control currents.
It is a further object of the present invention to provide an electronic thermostat which utilizes power from a heater activation circuit resulting in the elimination of alternate power sources for continuous operation of the thermostat.